The invention relates generally to the diagnosis and monitoring of the operation of electrical apparatus. More specifically, the present invention relates to a probe or sensor arrangement which facilitates the detection of flaws/imperfections in electrical apparatus such as stator cores of large generators, and which requires the stator to be energized only to a low level for detection purposes.
In the field of generating electricity on a commercial scale it is important that elements of the power generating system forming part of a 50˜1000-MVA power generating arrangement, for example, remain fully functional over their expected working life so that unexpected downtimes and/or catastrophic failures can be avoided. To avoid such problems it is important that elements such as the large stators, which form part of the above-mentioned generating systems be, carefully inspected and tested either during regular periodic maintenance or before being sold and installed in a power generating installation.
The stator core 30 of electric machines (such as schematically depicted in FIG. 1) utilizes thin insulated steel laminations 32 to reduce the eddy current flow for higher efficiency operation. The laminations are, as shown in FIGS. 2 and 3, stacked vertically by placing a dovetail grooves 34 of the laminations in the dovetail of a key bar 36, which is attached to the frame. To hold the laminations together and to prevent lamination vibration, the core is axially clamped with a force of about 300˜350 psi.
Shorting of the laminations 32 can be caused by manufacturing defects, damage during assembly/inspection/rewind, stator-rotor contact, vibration of loose coil wedges/laminations, foreign magnetic material, etc. If the laminations 32 are shorted for any reason, a larger circulating current is induced in the fault loop that consists of fault—laminations—key bar (see FIG. 2). The typical fault locations are shown in FIG. 3. The circulating fault current increases with the number of shorted laminations and the conductivity between the laminations and the short/key bar. The fault current increases the power dissipation in the stator core and causes localized heating. The hot spots can progress to more severe localized heating and eventually cause burning or melting of the laminations. As a result, the stator bar insulation and windings can also be damaged causing ground current flow through the stator core. Therefore, inter-laminar core faults should be detected and repaired to prevent further damage and to improve the reliability of generator operation.
In order to detect imperfections in the stator core construction various tests have been developed.
The so called “ring test” relies upon the detection of the eddy current heating caused by the short circuit currents. The generator core is wound with a number of turns (typically less than 10) of cable to form toroidal shaped excitation windings 31 in the manner schematically depicted in FIG. 1. The current level in the windings is chosen such that the flux driven in the core is near normal operating levels (approximately 1˜1.5 Tesla). The excitation requirement is at the several MVA level, since several hundred amperes and volts in the coil are needed to achieve the desired flux. The core 30 is operated in this manner for several hours. Thermal imaging cameras are then used to find “hot spots” on the inner stator surface. These hot spots indicate the location of the inter-lamination short circuits.
However, short circuits that are located below the surface of the stator teeth 37 and slots are difficult to find, since thermal diffusion causes the surface temperature rise to become diffuse/spread out. Because of the high power levels used in the ring test, personnel cannot enter the bore of the stator core during testing. Further, cables used in the test must be appropriately sized for the MVA level, which leads to long setup and removal times.
With this type of test, the high flux used is a cause for concern because: The high currents (100's of amperes and several KVs) are needed require a test supply capable of several MVA. The high current and voltage levels require care in the selection and installation of the excitation winding on the generator core and obscures parts of the core. Because the heating test is run on a core that is deprived of its normal cooling system, excessive heating can lead to core damage. The high current and voltage levels impact operator safety, and as mentioned above personnel are not allowed to enter the core interior when a ring test is running.
To overcome the above-mentioned shortcomings which tend to be encountered with the ring test, the so called “EL CID” (Electromagnetic Core Imperfection Detection) test was developed.
This test relies upon detection of the magnetic field caused by the short circuit currents that flow due to inter-lamination short circuits. As in the ring test, the generator core is wound with a number of turns in the manner of a toroid. The current level in the windings is chosen such that the core operates at approximately 4% of the normal operating flux. This corresponds to about a 5 volt/meter electric field induced along the core surface. The current requirement is in the 10-30 ampere range, so that a rather simple power supply of several kVA can be used. A magnetic potentiometer, referred to as a Chattock coil after its inventor, is used to sense the magnetic fields produced between two adjacent teeth by the short circuit currents that are induced in the inter-lamination insulation faults.
The Chattock coil (a.k.a. Maxwell worm or magnetic potentiometer) is used to sense the phase quadrature component of the magnetic field produced by any induced inter-laminar currents. Chattock coil voltages equivalent to those produced by a 100 mA or larger test current are used as the indicator for a severe inter-laminar short for the 4% flux excitation level.
The Chattock coil 38 typically spans the width of two adjacent teeth 37 in the manner shown in FIGS. 4 and 5 and is moved along the surface of the stator either by hand or by a robotic carriage. Because the short circuit current path is largely resistive, the magnetic flux created by the short circuit is in phase quadrature with the exciting flux. The signal from the Chattock coil is combined with a reference signal derived from the excitation current so that phase sensitive detection methods can be used to extract the fault signal from the background noise.
A fully digital EL CID system has been developed. This system exhibits improved noise suppression over the previous analog arrangements. Nevertheless, there are a number of anomalies and distortions which can arise when performing the EL CID test, and these must be interpreted using knowledge and experience of core construction.
The EL CID test involves exciting the core in a manner similar to that of the ring test, but uses much lower voltage and current levels. A flux of 4-5% is normal. The EL CID test procedure exhibits the following characteristics. The current required for this flux can be obtained from a variable transformer that is supplied from a standard electrical outlet. The induced voltage from this low flux is kept to about 5 volts/meter, so personnel can enter the core during the EL CID test to make observations. The induced currents at this flux are low enough not to cause excessive heating, so additional core damage due to testing is not a concern.
The EL CID test is better able to find inter-laminar faults which are located below the surface. This is a significant advantage over the ring test that relies upon thermal diffusion from the interior hot spot in order to provide detection. However, this test is such that the signal level in the coil results in high noise levels, especially when scanning in the end step region.
Another type of sensor arrangement is disclosed in USSR Inventor's Certificate No. RU 2082274 C1. This arrangement is directed to improving the sensitivity of the method and improving the interpretation and reliability of results. As depicted in FIGS. 6 and 7, it consists of a magnetizing winding 40, a device 42 for regulating the current in the winding and two sensors 44, 46. Each of these two sensors 44, 46 comprises, as shown in FIG. 6, a coil 48 wound onto a thin plate core 50 of ferromagnetic material with high magnetic permeability. The sensors 44, 46 have the same construction, however, one is used as a reference probe and the other is used as a scanning probe.
The outputs of the two sensors 44, 46 are connected to independent inputs of a phase shift device 52 which acts as a phase monitor and wherein the phase difference between the voltages of the two sensor units is used as a fault indicator. The outputs of the phase shift device are supplied to a personal computer (PC) or similar type of device via an A/D converter 54.
The two sensors 44, 46 can be supported on a triangular carriage arrangement 56 shown in FIG. 8 and thus moved through the interior of the stator under the control of a position control device 58, as shown in RU 2082274 C1.
However, this arrangement suffers from several drawbacks. It is sensitive to gap variation due to probe location. The magnitude and phase of the measured signal is very sensitive to the gap between lamination and probe (gap variation is an inherent limitation due to lamination surface roughness). That is to say:Magnitude of Vsense∝1/gap;Phase of Vsense∝tan−1(const/gap)
It is additionally difficult to scan and diagnose in the end step region (see FIG. 12 for example) of a stator core since the location makes it difficult to scan while maintaining a constant gap in the end step region.
Further, it is difficult, if not impossible, to construct a universal probe design. Since the teeth project inwardly into the interior from the inner cylindrical surface of the stator, they are inclined by a predetermined amount toward each other. Thus, the flat ferromagnetic core members 50 must be slightly angled and have a very shallow V-shape in order for each end of the RU 2082274 C1 arrangement to sit flat on the top of a tooth. With the change in diameter of a stator core, the angle and distance between the teeth vary and it is necessary for both the length and the angle of the shallow V-shape to vary. A change in the number of teeth also induces a change in the angle defined between the tops of two adjacent teeth, and thus induces the same problem.